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Eukaryotic Viruses

Eukaryotic Viruses

Viruses are submicroscopic infectious agents, which can replicate only inside the living cells of an organism. Viruses are composed of a nucleic acid genome surrounded by a protein coat, also known as capsid. The capsids of most eukaryotic viruses consist of a number of different proteins.  Viruses can infect both prokaryotic and eukaryotic organisms. Prokaryotic organisms such as bacteria can be infected by viruses, known as bacteriophages. Eukaryotic cells and their viruses carry out processes same as bacteriophages but the processes are not similar in all aspect, some processes are found only in eukaryotes and their viruses i.e., RNA processing and protein modification (proteolytic cleavage, glycosylation and phosphorylation). Many viruses are pathogenic and cause destruction of the host cell leading to disease in humans and other organisms.

 

Viruses infecting eukaryotic cells

Viruses with different nucleic acid genomes, which may be single or double stranded DNA or RNA infect eukaryotic cells. In some, the viral genome is a single molecule of nucleic acid whereas in others, the viral genome exist on more than one molecule and it is said to be segmented. Single-stranded (ss) genomes are replicated via a double-stranded (ds) intermediate and it may be positive sense or negative sense. The nucleic acid may be single-stranded or double-stranded, depending on the species. If the ssRNA is able to function as mRNA it is referred to as positive strand or plus strand RNA (+RNA); if it is the equivalent to antisense RNA it is known as minus strand or negative strand RNA  (-RNA). In some cases, the genome will encode mRNAs which are of either sense.

 

Structure of eukaryotic viruses

The viral genome is surrounded by a protein shell known as capsid. Capsid encloses the genetic material of the virus and consists of protein subunits known as capsomeres. The nucleic acid genome plus the protective protein coat is called the nucleocapsid which may have icosahedral, helical or complex symmetry. The majority of viruses have capsids with either helical or icosahedral structure. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere. All faces of an icosahedron are identical. In the icosahedral structure, the individual polypeptide molecules form a geometrical structure that surrounds the nucleic acid (Figure-1).

 

Figure 1: Structure of Icosahedral viruses
Figure 1: Structure of Icosahedral viruses

 

Adenovirus has an icosahedral structure. In the helical or filamentous structure, the polypeptide units are arranged as a helix to form a rod like structure surrounding the nucleic acid genome. Tobacco mosaic virus has a helical structure (Figure-2).

 

Figure 2: The Tobacco mosaic virus showing helical structure
Figure 2: The Tobacco mosaic virus showing helical structure

      

       Some viruses are enveloped (Figure-3), where the capsid is coated with a lipid membrane also known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane derived from the host cell as the virus is released from the cell.  In some cases, the virus buds through the plasma membrane but in other cases the envelope may be derived from internal cell membranes such as those of the Golgi body or the nucleus.

 

Figure 3: Basic structure of an enveloped virus
Figure 3: Basic structure of an enveloped virus

 

       Some viruses bud through specialized parts of the plasma membrane of the host cell; for example, Ebola virus associates with lipid rafts that are rich in sphingomyelin cholesterol and glypiated proteins. Poxviruses are exceptional in that they wrap themselves in host cell membranes using a mechanism that is different from the usual budding process used by other viruses. Enveloped viruses do not necessarily have to kill their host cell in order to be released, since they can bud out of the cell. If the envelope remains intact, enveloped viruses behave as readily infectious agents.

       The viral glycoproteins play an important role in facilitating infection by interacting with receptor proteins on the surface of the host cell. Once the virus has infected the cell, it will start replicating itself, using the mechanisms of the infected host cell. During this process, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell.

 

DNA viruses

A DNA virus is a virus that contains DNA as its genetic material and it replicates using a DNA-dependent DNA polymerase. The DNA may be single-stranded or double-stranded and may be linear or circular. Most of the DNA viruses replicate in the nucleus and with the help of the host cell machinery replicate their genomes and express viral genes.

       Some viruses have circular genomes  whereas others have linear genomes. The eukaryotic viruses having circular double-stranded genomes include baculoviruses, papovavirues and Polydnaviruses. The eukaryotic viruses having linear double-stranded genomes include adenoviruses, Herpes viruses. Parvoviruses are very small, single-stranded non-enveloped DNA viruses. 

 

Baculoviruses

Baculoviruses are very small viruses having double-stranded circular DNA as their genetic material. Baculoviruses are pathogens that attack insects and other arthopods. The baculoviruses can be divided to two genera: nucleopolyhedroviruses (NPV) and granuloviruses (GV). While granuloviruses contain only one nucleocapsidper envelope, NPVs contain either single (SNPV) or multiple (MNPV) nucleocapsids per envelope. These viruses are excellent candidates for species-specific, narrow spectrum insecticidal applications. They have been shown to have no negative impacts on plants, mammals, birds, fish, or even on non-target insects.

       There are two distinct forms of baculovirus exist in its lifecycle, i.e., occlusion derived virus (ODV) which is responsible for the primary host infection and the budded virus (BV) which is released from the infected host cells later during secondary infections. Baculovirus infects in three phases i.e., early phase, late phase and very late phase. The budded virus is produced in the late phase,  and the occlusion derived virus form is produced in the very late phase acquiring the envelope from host cell nucleus and embedded in the matrix of occlusion body protein.  The infection begins when a susceptible host insect feeds on plants which are contaminated with the occluded form of the virus. The occlusion derived virus is present in a protein matrix. The protein matrix dissolves in the alkaline environment of the host midgut, releasing ODV that then fuse to the columnar epithelial cell membrane of the host intestine and are taken into the cell in endosomes.  Nucleocapsids come out  from the endosomes and are transported to nucleus. Viral transcription and replication occurs in the cell nucleus and new BV particles are budded out from the basolateral side to spread the infection systemically. During budding, BV acquires a loosely fitting host cell membrane with expressed and displayed viral glycoproteins. 

 

Papovaviruses

Papovaviruses are viruses containing double-stranded DNA, are icosahedral in shape, and do not have a lipoprotein envelope. All papovaviruses replicate inside nucleus. The Papovavirus family is one of the many virus families associated with human disease. The papovaviruses have oncogenic potential. The Papovavirus is divided into two subfamilies or genera, Polyomavirus and Papillomavirus. They are commonly found in humans and other species, mostly mammals.

       Polyomaviruses exhibit asymptomatic persistent infections in humans. Polyomaviruses are icosahedral, 45-nm diameter particles, with three capsid proteins and without an envelope. They contain a 5-kbp circular, double-stranded DNA genome. These DNA viruses having small genomes are more dependent on the host cell for replication and gene expression. Human and animal polyomaviruses are antigenically distinct. The monkey virus SV40, which has a tumour causing ability is having a small genome of 5kbp which contains five genes. The genes are accommodated in the small genome by being present on both strands of the DNA and by having overlapping sequences. Viral proteins are produced by a combination of the use of overlapping reading frames and alternative splicing. The genes are expressed in two phases after infection and can be categorised into early and late genes. The early genes produce proteins that activate transcription called the large T antigen and the small T antigen. The large T antigen is the critical gene product in the SV40 system. The ability of large T antigen to bind cellular p53 and Rb family proteins is required for SV40 transforming activity.  These stimulate both viral and host cell transcription and are responsible for the tumorigenic properties of the virus.

       Human polyomaviruses establish persistent infections in the kidneys; these infections may reactivate in immunosuppressed hosts and during some normal pregnancies. Progressive multifocal leukoencephalopathy is a rare demyelinating disease of the central nervous system of some immunosuppressed patients. It is caused by replication of JC virus in oligodendrocytes. Although oncogenic in rodents, polyomaviruses are not believed to be important factors in human cancers. BK and JC types of polyomavirus are widespread. Infections occur during childhood, and 70 to 80 percent of adults have antibodies. The route of transmission may be respiratory.

       Papillomaviruses were first identified in the early 20th century, when it was shown that skin warts, or papillomas, could be transmitted between individuals by a filterable infectious agent. Papillomaviruses have been found in many species, including humans, rabbits, cows, and dogs. In recent years, HPV, in particular, has become a major source of concern as it has been consistently linked to genital cancer.  Papillomaviruses are similar to polyomaviruses , but here the genome is 8kbp in size and the particles are 55-60nm in diameter.

       The genome organization of the papillomaviruses differs significantly from that of the polyomaviruses. In papillomavirus, all the genes are encoded in a single DNA strand.  Nucleotide sequence analyses have revealed at least six open reading frames. Two are “late” open reading frames that encode capsid proteins; others are “early” open reading frames that are involved in viral replication and cellular transformation. In the figure (Figure-4), it shows that the papillomavirus genome is divided into an early region (E), encoding various genes that are expressed immediately after initial infection of a host cell, and a late region (L) encoding the capsid genes L1 and L2. E1encodes a protein that binds to the viral origin of replication in the long control region of the viral genome.

 

Figure 4: Genome organization of Human Papillomavirus type16
Figure 4: Genome organization of Human Papillomavirus type16

       

       The E2 protein serves as a master transcriptional regulator for viral promoters located primarily in the long control region. E2 facilitates the binding of E1 to the viral origin of replication. The E3 gene is not known to be expressed as a protein and does not appear to serve any function. The E4 protein is expressed at a low level during early stage of viral infection, but expression of E4 increases very much during late phase of viral infection. The E5 are small, very hydrophobic proteins that destabilise the function of many membrane proteins in the infected cell. The E5 proteins of human papillomaviruses associated to cancer, however, seem to activate the signal cascade initiated by epidermal growth factor upon ligand binding. HPV16 E5 and HPV2 E5 have also been shown to down-regulate the surface expression of major histocompatibility complex class I proteins, which may prevent the infected cell from being eliminated by killer T cells. The E6 protein appears to have multiple roles in the cell and it interacts with many other proteins. One of the most important role of the E6 protein is to  mediate the degradation of p53, a major tumour suppressor protein, reducing the cell's ability to respond to DNA damage. It is also responsible for altering several metabolic pathways by targeting other cellular proteins. The primary function of the E7 protein is to inactivate members of the pRb family of tumour suppressor proteins. The protein E7 and E6 together prevent cell death and promote cell cycle progression. The E8 gene encodes a small protein in some papillomaviruses.  

       Papillomaviruses are usually considered as highly host- and tissue-tropic, and are thought to rarely be transmitted between species. Human papillomavirus types are distinguished by DNA hybridization assays or DNA sequence analysis. There are more than 70 human types of papillomaviruses. Papillomaviruses replicate exclusively in the basal layer of the body surface tissues. All known papillomavirus types infect a particular body surface, typically the skin or mucosal epithelium of the genitals, anus, mouth, or airways.

       Papillomaviruses replicate exclusively in keratinocytes. Keratinocytes form the outermost layers of the skin, as well as some mucosal surfaces, such as the inside of the cheek or the walls of the vagina. These surface tissues, which are known as stratified squamous epithelia, are composed of stacked layers of flattened cells. The cell layers are formed through a process known as cellular differentiation, in which keratinocytes gradually become specialized, eventually forming a hard, crosslinked surface that prevents moisture loss and acts as a barrier against pathogens. Less-differentiated keratinocyte stem cells, replenished on the surface layer, are thought to be the initial target of productive papillomavirus infections.

 

Adenoviruses

Adenoviruses were first isolated in 1953 from human adenoids. Adenoviruses are  the non-enveloped, viruses having double-stranded linear DNA, varying in size between 90-100nm.  Adenoviruses are common viruses which can infect various species of vertebrates, including humans and normally the infections are  asymptomatic. Adenoviruses are a frequent cause of acute upper respiratory tract infections, i.e., cold. There are 57 described serotypes in humans,  which  can cause fever, diarrhoea, conjuctivitis, bladder infection and many other infections. It is transmitted via direct inoculation to the conjunctiva, a fecal-oral route, aerosolized droplets, or exposure to infected tissue or blood.

       Adenoviruses are larger in size and so they can be transported through the endosome. This virus has a unique spike associated with each penton base of the capsid  (Figure-5) which helps in the virus attachment with the host cell through the coxsackie-adenovirus receptor on the surface of the host cell. 

 

Figure 5: Structure of an Adenovirus
Figure 5: Structure of an Adenovirus

 

       Adenoviruses can replicate in the nucleus of mammalian cells using the host's replication machinery.  The knob domain of the fiber protein binding to the host cell receptor initiates the entry of the virus into the host cell. After the virus enters into the host cell, the endosome acidifies and the capsid components disassociate which cause the release of the virion into the cytoplasm. The toxic nature of the pentons also help in the release of the virion into the cytoplasm. Then the virus is transported to the nuclear pore complex with the help of cellular microtubules. The adenovirus particle disassembles at the nuclear pore complex and the viral DNA is released which enters the nucleus through the nuclear pore. Then the DNA is associates with histone molecules. And with the expression of the viral gene new virus particles can be generated.

       Replication of adenoviruses is divides into early and late phases.  Transcription of the adenovirus genome is regulated by virus-encoded trans-acting regulatory factors. Early genes are encoded at various locations on both strands of the DNA  and are responsible for expressing mainly non-structural, regulatory proteins. These regulatory proteins can alter the expression of host proteins that are necessary for DNA synthesis, activate other viral genes and avoid permature death of the infected cell by the host-immune defenses. Multiple protein products are made from each early gene by alternative splicing of mRNA transcripts. The first protein to be made is E1A, which is a trans-acting transcriptional regulatory factor which is necessary for transcriptional activation of early gene. The second protein is E1B and  E1A and E1B together can transform primary cells in vitro.  DNA replication separates the two phases, i.e., early and late phases. Replication of the adenovirus genome occurs after the liberation of sufficient viral proteins, replication machinery and replication substrates by the early genes. The late phase of the adenovirus lifecycle mainly produces sufficient quantities of structural protein to pack all the genetic material produced by DNA replication. Once the viral components have successfully been replicated, the virus is assembled into its protein shells and released from the cell as a result of virally induced cell lysis.

 

Herpes viruses

Herpes viruses are large, icosahedral, enveloped viruses possessing linear double-stranded DNA as their genetic material. These viruses infect a range of vertebrates, including humans, where they cause diseases such as chicken pox, shingles and glandular fever.

       Herpes simplex virus 1 and 2 (HSV-1 and HSV-2) are two members of the herpes virus family, which infect humans. Herpes simplex virus-1 (HSV-1) is a well-studied herpes virus that causes cold sores. HSV-1 and HSV-2 are transmitted from contact with an infectious area of the skin during reactivations of the virus. Although less likely, the herpes viruses can be transmitted during latency.  HSV-1 is usually acquired orally during childhood, but may also be sexually transmitted. HSV-2 produces most genital herpes. Both HSV-1 and HSV-2 are ubiquitous and contagious in nature. HSV-1 and HSV-2 each contain at least 74 genes within their genomes. HSV-1 has a 150kbp double-stranded DNA genome which contains the genes present on both DNA strands, some overlapping with each other.  These genes encode a variety of proteins involved in forming the capsid, tegument and envelope of the virus, as well as controlling the replication and infectivity of the virus. The transcription and replication of the viral genome are tightly regulated. Transcription of HSV genes is catalyzed by RNA polymerase II of the infected host. The viral genes fall into three groups called immediate early or α genes, early or β genes and late or γ genes which are expressed in a defined sequence following infection of a host cell. Immediate early genes, which encode proteins that regulate the expression of early and late viral genes, are the first to be expressed following infection. Early gene expression follows, to allow the synthesis of enzymes involved in DNA replication and the production of certain envelope glycoproteins. Expression of late genes occurs last as this group of genes predominantly encode proteins that form the virion particle. Five types of proteins i.e., UL6, UL18, UL35, UL38 and UL19 form the capsid and among the five capsid proteins, UL19 is the major capsid protein.

       The interaction of several glycoproteins on the surface of the virus with the receptors on the surface of the host cell make an entry of the virus into the host. The envelope covering the virus, interacts with the host cell receptors and it fuses with the host cell membrane and simultaneously creates an opening through which the virus gets an entry into the host cell. In this process, the virus releases the nucleocapsid directly into the host cytoplasm. After its entry in the host cytoplasm,  it is transported into the host nucleus, and once attached to the nucleus at a nuclear entry pore, the capsid ejects its DNA contents via the capsid portal. The capsid portal is formed by twelve copies of the capsid protein UL6 arranged as a ring. In the nucleus, the viral DNA is transcribed into early mRNAs which are transported to the cytoplasm for the translation of early proteins. These early proteins are brought back into the nucleus and participate in the replication of the virus DNA into many copies. The viral DNA is then transcribed into the late mRNAs which exit to the cytoplasm for translation into the late proteins. The capsid proteins encapsidate the newly replicated genomes. The envelope proteins are embedded in the nuclear membrane. The nucleocapsid are enveloped by budding through the nuclear membrane, and the mature viruses are released from the cell through cytoplasmic channels.

 

RNA viruses  

RNA viruses have RNA as their genetic material. The genomes of RNA viruses may be single-stranded or double-stranded. Single-stranded RNA viruses are may be of positive sense or negative sense. Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell. Poliovirus (Family-Picornaviruses), togaviruses, flaviviruses are some examples of positive-sense RNA viruses. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. Influenza virus (Family-Orthomyxoviruses) , measles virus (Family-Paramyxoviruses), rabies virus (Family-Rhabdoviruses) are some examples of negative-sense RNA viruses. Double-stranded RNA viruses need to package an RNA polymerase to make their mRNA after infection of the host cell. Rotaviruses belong to family Reoviruses are double-stranded RNA viruses. RNA viruses that copy their RNA into DNA are known as retroviruses. These viruses are single-stranded positive-sense RNA viruses. The genome of retrovirus is though single-stranded, it doesn't function as mRNA immediately on infection, because it is not released from the capsid into the cytoplasm. It serves as a template for the enzyme reverse transcriptase and is copied into DNA.

       Transcription of viral genes from an RNA genome requires enzyme RNA dependent RNA polymerases which are not present in host cells but are encoded by a viral gene known as pol. Unlike DNA viruses, transcription and replication of RNA viruses do not take place in the nucleus of host cell, as they are independent of host cell polymerases. Most of the RNA viruses replicate in the cytoplasm. RNA dependent RNA polymerases are not capable of proofreading and replicate their templates with a higher error rate. The mutation rate of RNA viruses is very higher than that of DNA viruses as RNA viruses are capable of evolving rapidly and can develop changes in antigenicity and virulence quickly allowing them to adapt to changing environments and attempts to eliminate them by the host's immune system.

 

Picornaviruses

Picornaviruses are small, nonenveloped viruses with icosahedral symmetry containing a single positive strand RNA genome. Picornaviruses are divided into two groups, i.e., Enteroviruses and the Rhinoviruses. Poliovirus, Coxsackievirus and Echovirus  are the  member of Enteroviruses.

       The Enteroviruses enter via the intestinal tract and attach to receptors on intestinal epithelia. The virus replicates in the cytoplasm. The virus enters the bloodstream by passing through the lymphatic circulation. Picornaviruses commonly produce subclinical infections; acute disease may range from minor illness to paralytic disease.

       Three types of polioviruses, i.e., type-1, type-2 and type-3 polioviruses are recognised. Their genome contain a 7000 base positive strand RNA. These viruses adsorb only to intestinal epithelial cells and motor neuron cells of the central nervous system. Poliovirus virion RNA functions as an mRNA but does not have the methylated cap structure typical of eukaryotic mRNAs. It has an internal ribosome entry site (IRES) which enables ribosomes to bind without having to recognize a 5' methylated cap structure. The mRNA is translated into a single polypeptide (polyprotein), which is cleaved. The cleavages occur before translation is complete  i.e. on the growing polypeptide chain and are carried out by virally coded proteases. Viral RNA polymerase copies plus-sense genomic RNA into complementary minus-sense RNA. New minus sense strands serve as template for new plus sense strands. The new plus strand can serve as a template for more minus strands, or it may be packaged into progeny virions or it may be translated into polyprotein.

 

Orthomyxoviruses

The Orthomyxoviruses are a family of single-stranded RNA viruses of negative sense that is segmented. This group of viruses  include five genera, i.e., Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus and Thogotovirus. The influenzavirus A, B, and C cause influenza in vertebrates. Whereas Isaviruses infects salmon and thogotoviruses infects both vertebrates and invertebrates. Influenzavirus A infects humans, birds and other mammals, Influenzavirus B infects humans ans seals, Influenzavirus C infects humans and pigs.

       The influenza virion is pleomorphic, i.e., the envelope can occur in spherical and filamentous forms. In general, the virus's morphology is spherical with particles 50 to 120 nm in diameter, or filamentous virions 20 nm in diameter and 200 to 300  nm long. There are some 500 distinct spike-like surface projections of the envelope each projecting 10 to 14 nm from the surface with some types densely dispersed over the surface, and with others spaced widely apart. The influenza A virus particle is 80-120 nm in diameter and usually roughly spherical, although filamentous forms can occur. The influenza A genome is not a single piece of nucleic acid, instead it contains eight pieces of segmented negative-sense RNA which encode eleven proteins.  Hemagglutinin and neuraminidase are the two important glycoproteins found on the outside of the viral particels. These two glyocoproteins are the target of antiviral drugs. Neuraminidase is an enzyme involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles, whereas hemagglutinin is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell. The interior of the virion also contains another protein called NEP (Figure-6).

 

Figure 6: Structure of an Influenza virus (Orthomyxovirus)
Figure 6: Structure of an Influenza virus (Orthomyxovirus)

      

       The Influenza virus is typically transmitted from infected mammals through the air by coughs or sneezes and it is also transmitted by nasal secretions, saliva, faeces and blood. Flu viruses can remain infectious for about one week at human body temperature, and indefinitely at very low temperatures. The viruses bind to a cell through interactions between its hemagglutinin glycoprotein and sialic acid sugars on the surfaces of epithelial cells in the lung and throat. Through endocytosis, the cell imports the virus. In the acidic endosome, part of the haemagglutinin protein fuses the viral envelope with the vacuole's membrane, releasing the viral RNA molecules, accessory proteins and RNA-dependent RNA polymerase into the cytoplasm. These proteins and viral RNA form a complex that is transported into the cell nucleus, where the RNA-dependent RNA polymerase begins transcribing complementary positive-sense cRNA. The cRNA is either exported into the cytoplasm and translated, or remains in the nucleus. Newly-synthesised viral proteins are either secreted through the Golgi apparatus onto the cell surface or transported back into the nucleus to bind viral RNA and form new viral genome particles.   

 

Paramyxoviruses

The paramyxoviruses are pleomorphic, enveloped, having negative-sense, non-segmented single-stranded RNA as their genetic material. They have a helical nucleocapsid. The envelope surrounding the virus contains two virally coded glycoproteins, i.e., the fusion protein and the attachment protein. Fusion proteins and attachment proteins appear as spikes on the virion surface (Figure-7). The F protein has fusion activity whereas the attachment protein binds to receptors on the host cell. The attachment protein may have hemagglutinating (ability to cause red blood cells clump) activity and neuraminidase activity (ability to cleave sialic acid on the cell surface) or only having hemagglutinating activity or none of them. Matrix proteins inside the envelope stabilise virus structure.

 

Figure 7: Structure of a Paramyxovirus
Figure 7: Structure of a Paramyxovirus

 

       The nucleocapsid core is composed of the genomic RNA, nucleocapsid proteins, phosphoproteins and polymerase proteins. The nucleocapsid protein associates with genomic RNA  and protects the RNA from nuclease digestion. The phosphoprotein binds to the nucleocapsid proteins and large proteins and forms part of the RNA polymerase complex. The matrix protein organizes and maintains the structure of the virion. The fusion protein projects from the envelope surface as a trimer, and mediates cell entry by inducing fusion between the viral envelope and the cell membrane by class I fusion. The large protein is the catalytic subunit of RNA-dependent RNA polymerase.

       A number of important human diseases are caused by paramyxoviruses. Mumps, measles are the diseases caused by the paramyxoviruses. The parainfluenza viruses are the second most common causes of respiratory tract disease in infants and children. They can cause pneumonia, bronchitis and croup in children and the elderly. They also cause diseases in animals, birds and other mammals.

       The virus multiplies inside the host cytoplasm. The viral RNA polymerase uses the nucleocapsid as the template. Viral mRNAs are transcribed, capped, methylated and polyadenylated. The viral mRNAs are translated to give viral proteins. RNA polymerase and RNA modification enzymes are packaged in the virion. Viral RNA replication involves full length plus strand synthesis. Plus strand is used as a template for full length minus strand. New full length minus strands may serve as templates for replication, or templates for transcription, or they may be packaged into new virions. The attachment protein and fusion protein both are translated as transmembrane proteins and transported to the cell plasma membrane.

 

Reoviruses

The reoviruses are non-enveloped, and have double-stranded segmented RNA as their genetic material. They have an icosahedral capsid, composed of an outer and inner protein shell (Figure-8). These viruses can infect gastrointestinal system and respiratory tract. Reovirus infection occurs often in humans, but generally these are mild infections. The virus can be readily detected in faeces, and may also be recovered from pharyngeal or nasal secretions, urine, cerebrospinal fluid, and blood. Some viruses of this family infect plants.

 

Figure 8: Structure of a Reovirus
Figure 8: Structure of a Reovirus

      

       Rotavirus is a genus in the family reoviruses. Group A rotaviruses are major pathogens in humans and animals. Rotaviruses have a distinctive wheel-like shape. Complete particles have a double-layered capsid and measure about 70 nm in diameter. The rotavirus genome contains 11 segments of double-stranded RNA. The segmented genome of rotavirus readily under goes genetic reassortment during co-infection. Rotaviruses replicate exclusively in the cytoplasm. The virion enters the cell by endocytosis, and the outer shell of the double capsid is removed in lysosomes with the liberation of 50-nm subviral particles, thus activating the viral RNA polymerase. The mRNAs are made by virally-coded RNA polymerase packaged in the virion. The RNA is capped and methylated by virion packaged enzymes. It is then extruded from the vertices of the capsid. The mRNAs are translated and the resulting viral proteins assemble to form an immature capsid. The mRNAs are packaged into the immature capsid and are then copied within the capsid to form double stranded RNAs. About 8 hours after infection, viroplasmic inclusions of dense granular material, representing newly synthesized proteins and RNA, accumulate in the cytoplasm. Viral RNA is packaged into core particles, and viral capsid proteins assemble around the cores. These particles accumulate in vesicles of the endoplasmic reticulum and leave the viroplasm by budding through the membrane of the endoplasmic reticulum, where they acquire the outer capsid protein.

 

Retroviruses

Retroviruses are the RNA viruses with single-stranded positive sense RNA genomes. Retroviruses contain two copies of the genome in each viral particle. When they infect a host cell, the single-stranded RNA is converted into double-stranded DNA copy by the enzyme reverse transcriptase. The double-stranded DNA is integrated into the host cell genome by a viral integrase enzyme. The integrated form is also known as provirus and it acts as the template for replication of the viral genome and expression of the viral genes. Human immunodeficiency virus (HIV) is a retrovirus which causes the acquired immunodeficiency deficiency syndrome (AIDS).  

To know more about Retroviruses refer the article ''Retroviruses and Reverse transcriptase'' inside discoverbiotech wiki.
 

References

1. Fletcher, Hugh etal. (First Indian reprint,2010). Genetics, Bios Instant notes (128-131), 3rd Edn. Taylor & Francis Group.

2. Murray et.al., Medical Microbiology, 6th Edn. Elsevier Inc.

3. www.ncbi.nlm.nih.gov

4. www.wikipedia.org






 

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